Girlfriend compatible route in the Lammermuir hills needed

We're off for a week near Eyemouth soon and I'm thinking we could take the bikes and drive out to the Lammermuirs one day. The YouTube videos I've seen look ideal for her skill level (mellow landrover tracks, nothing too technical!)

I've got the OS map and hunted for GPX's but struggling to find a nice 15-20 miler. It's also my first time planning a ride in Scotland and confused by the lack of ROW restrictions!

As regards restrictions - there are non so long as you behave responsibly! Marked as doubletrack on the map it will be rideable and could be anything from easy gravel road to faint old cart track. Single track marked ont eh map culd be anything from lovely old path to " path - What path?"